Effective thermal management is a key factor in ensuring stable electronic device performance over a long lifetime. For electronic devices, a high operating temperature can reduce the lifetime of the devices and their efficacy. In addition, for optoelectronic devices, for example light-emitting diodes (LEDs), the junction temperature thereof can also influence the wavelength of the emitted light. Therefore, effective thermal management of these electronic devices is required.
Adequate cooling may not be achieved by mounting high-powered electronic components to standard laminate boards, for example FR4 boards. This form of board typically does not provide sufficient thermal conductivity to remove heat from high-powered components in order that they can operate within a desired temperature range. As a result, secondary cooling systems for example, heatsinks or coldplates are often used in conjunction with these laminate boards. While adding a secondary cooling system provides an improvement in thermal management, the thickness of a laminate board can provide a barrier to thermal conductivity.
Incorporating thermal management into printed circuit boards (PCBs) has enhanced the thermal flow between the heat source and the cooling system, resulting in improved thermal management. PCBs may include thermal vias comprising thermally conductive materials such as copper or aluminium that are placed in direct thermal contact with heat-producing components. In metal-core PCBs (MCPCB), for example, the core of the board comprises a thermally conductive metal. An MCPCB can be effective because it can be provide close proximity between heat-producing electrical components and the thermally conductive material, however, the thermal properties of such modified PCB boards are typically insufficient for many of today's applications. Hence, more advanced thermal management systems for use with high-powered electronic components have been developed in order to meet this need.
For example, heat pipes, thermosyphons and other two-phase cooling devices have been designed to remove heat from high-power electronic components in an efficient manner. In these devices, heat is transported away from the heat source by means of a heat conducting fluid inside the device. This device typically has two ends, namely an evaporator end and a condenser end. At the evaporator end the fluid evaporates upon absorption of the heat, travels to the condenser end, and condenses upon release of the heat, wherein this fluid may be water or some other evaporable fluid. Heat pipes and thermosyphons are passive systems, thereby requiring no drive circuitry or moving parts to enable their operation. These devices have proven to be effective in moving heat away from high-powered electronic components, particularly when paired with a secondary cooling system. However, these devices are typically designed to be in contact with metal-core PCBs or other substrates that, while being thermally conductive, typically do not enable thermal management as effectively as the heat pipes. As such, benefits of a heat pipe are typically not optimized, as there is a thickness of a less thermally conductive substrate between the heat-producing element and the heat pipe.
A number of literature references disclose the use of thermally conductive devices for use with a heat sink apparatus. For example, U.S. Pat. No. 4,106,188 discloses a package that uses direct cooling of high power transistors by incorporating the components into a heat pipe. The devices are mounted on the inside wall of a heat pipe such that they become part of the wall structure. Electronic circuitry is included, however it does not allow for complete functionality of the devices. In addition, the invention does not discuss how to effectively thermally manage mounted optoelectronic devices for example LEDs or lasers, which are mounted on an exterior surface.
U.S. Pat. No. 6,573,536 and United States Patent Application Publication No. 2004/0141326 disclose a light source comprising LEDs mounted to the side of a hollow thermally conductive tube that uses air as the cooling medium wherein the air flows in one direction inside the tube. Electrical connections to the LEDs can be achieved through conductive paths disposed on an electrically insulating layer. These conductive paths can be provided by means of one or more flexible printed circuits that are placed on the surface of the tube. The means of placing the flexible printed circuits on the surface of the tube however, is not disclosed. Specifically in this prior art the thermal management design and the electrical subsystem are conceived as two separate components and not as one integrated system.
International Publication No. WO 03/081127 discloses a Cooled Light Emitting Apparatus that utilizes a combination of heat pipe and thermoelectric coolers to dissipate heat created by high power LEDs. The LEDs are mounted on a heat spreader plate, which is in thermal contact with a thermoelectric cooler, and which passes the heat to a heat pipe or other heat exchange system. For this system, the thermoelectric cooler requires a current passed through it in order to activate the cooling function, which can result in addition operational power of this system.
United States Patent Application Publication No. 2001/0046652 discloses a Light Emitting Diode Light Source for Dental Curing. This publication discloses simple circuitry in the form of one electrically conducting layer and one electrically insulating layer that are deposited on one side of a thermally conductive substrate possessing machined trenches that are used to create simple circuitry. The substrate is in contact with a thermally conductive member such as a heat pipe. The LEDs are mounted directly to the substrate, assuming it to be electrically conductive. Control electronics and LEDs are separated and no reference is made to mix accompanying electronics with high-power devices on a single substrate.
International Publication Nos. WO 2004/038759 and WO 2004/011848 disclose a method and apparatus for using light emitting diodes for curing composites and various solid-state lighting applications. In this invention, one or more LEDs are mounted either directly on a heat pipe or on a substrate that is in thermal contact with the heat pipe. The invention discloses integrating circuitry through substrate patterning and through the utilization of printed circuit boards in close contact with the heat pipe.
United States Patent Application Publication No. 2004/0120162 discloses a light source that may be used as part of a dental curing lamp. It discloses LED dies that are placed on a substrate that is in contact with a heat exchanger. However, there is no discussion of the integration of electronic circuitry necessary to drive the LEDs.
U.S. Pat. No. 5,216,580 discloses an optimized integral heat pipe and electronic circuit module arrangement. This patent discloses a ceramic substrate carrying electronic components on one side and metallization and a wick structure on the opposing side. The heat pipe comprises an attached matching structure containing a vapour chamber filled with evaporative fluid. The substrate material of this invention is limited to ceramics, and this invention is also limited to the placement of specific electronic devices on such a heat pipe.
While there are many electronic device substrates that incorporate highly thermally conductive systems, the design of such substrates is essentially planar which limits the number of components per useable substrate area that can be thermally managed. Therefore, there is a need for a new apparatus that unifies thermal conductivity and electrical conductivity with an added possibility for enhanced package densities.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.